1. Field of the Invention
The present invention relates to an electrophotographic image formation apparatus, such as an analog copying machine, a digital copying machine, and a printer.
2. Discussion of Background
Conventionally, the following photoconductors are known as electrophotographic photoconductors for use in electrophotographic image formation apparatus:
(1) a single layer photoconductor comprising an electroconductive support 21 and a single photoconductive layer 23 provided on the electroconductive support 21 as schematically shown in FIG. 3;
(2) a function-separated type layered photoconductor for use with incoherent writing light, comprising an electro-conductive support 21, and a layered photoconductive layer comprising a charge generation layer (CGL) 31 and a charge transport layer (CTL) 32, provided on the electroconductive support 21 as schematically shown in FIG. 4; and
(3) a function-separated type layered photoconductor for use with coherent writing light, comprising an electro-conductive support 21, an undercoat layer (UL) 25 provided on the electroconductive support 21, and a layered photoconductive layer comprising a charge generation layer (CGL) 31 and a charge transport layer (CTL) 32, provided on the undercoat layer (UL) 25 as schematically shown in FIG. 5.
The single layer photoconductor has a simple structure and is inexpensive. However, a sufficiently high photosensitivity for use in practice is difficult to obtain.
The function-separated type layered photoconductor for use with incoherent writing light as shown in FIG. 4 does not include an undercoat layer (UL) or includes an extremely thin undercoat layer (UL), which is as thin as 1 xcexcm or less.
The function-separated type layered photoconductor for use with coherent writing light as shown in FIG. 5 includes an undercoat layer (UL), but the undercoat layer (UL) is still thin.
In the function-separated type layered photoconductor for use with coherent writing light, unless the undercoat layer (UL) 25 is provided, multiple reflections of the coherent writing light is caused between the charge transport layer (CTL) 33 which is the top layer of the photoconductor and the electroconductive layer 21, and the interference of the coherent writing light subjected to multiple reflections causes uneven photosensitivity in the photoconductor, with the formation of adverse interference fringes in reproduced images. In order to prevent the interference, the undercoat layer (UL) 25 is provided. The thickness of the undercoat layer (UL) 25 is several xcexcm, preferably 5 xcexcm or less. The charge transport layer (CTL) 33 usually has a thickness of 20 xcexcm to 40 xcexcm.
In comparison with the single layer photoconductor as shown in FIG. 3, the function-separated type layered photoconductors as shown in FIG. 4 and FIG. 5 have a higher photosensitivity and are mainly used in the field of the currently employed organic photoconductors. However, the charge generation layer (CGL) 31 is provided under the charge transport layer (CTL) 33, so that electric charges generated by the writing light diffuse transversely, repelling each other due to the electric field of each charge in the course of the transfer of the electric charges through the charge transfer layer (CTL) 33. This causes the formation of blurred latent electrostatic images, and accordingly blurred toner images when developed.
It is considered that a method of reducing the thickness of the CTL to 20 xcexcm or less, preferably 15 xcexcm or less could be effective for preventing the formation of such blurred images. However, when the thickness of the CTL is reduced to 20 xcexcm or less, the electrostatic capacity of the photoconductor becomes so large that a sufficient surface potential for image formation cannot be obtained. This phenomenon becomes conspicuous in particular when the thickness of the CTL is 15 xcexcm or less. This is because the withstand electrostatic voltage of the CTL is 40 V/xcexcm to 50 V/xcexcm, so that the photoconductor should be used at 30 V/xcexcm or less to be on the safe side. Therefore, it is preferable that a photoconductor with a thickness of 20 xcexcm be used by being charged to 600 V or less, and with respect to a photoconductor with a thickness of 15 xcexcm, it is preferable that the charging thereof be 450 V or less. In most image formation apparatus in which a cleaning blade is used for cleaning the photoconductor, the thickness of the photoconductor is generally increased to 30 xcexcm to 40 xcexcm with a margin scraped from the surface thereof by the cleaning blade taken into consideration.
The undercoat layer (UL) for use in the conventional photoconductor for use with the coherent writing light can take over a partial voltage of only about 20% of the voltage with which the photoconductor is charged, which partial voltage is not sufficient for preventing the electrostatic breakdown of the photoconductor.
Furthermore, by reducing the thickness of the CTL, there can be prevented the diffusion of electric charges generated within the CGL, which is caused by the mutual repulsion of the electric field of each electric charge in the course of the transfer of the electric charges through the CTL. However, in the case where the thickness of the CTL is merely reduced, the surface potential of the photoconductor decreases if electric charges are applied thereto under the same conditions as in the case where the thickness of the CTL is not reduced, and therefore satisfactory development cannot be carried out by merely reducing the thickness of the CTL. Some conventional photoconductor with such a CTL with a reduced thickness is not capable of clearly reproducing low-contrast thin line images, or utterly unable to produce such line images.
In conventional photoconductors for analog writing, the UL has such a thickness that is sufficient for preventing charge injection from the electroconductive support into the CGL and the CTL, or for making it possible to perform the pretreatment of the electroconductive support for uniformly providing the CGL on the electroconductive support by coating. Therefore, the undercoat layer is extremely thin, with By reducing the thickness of the charge transport layer, there can be prevented the diffusion of electric charges generated within the charge generation layer, which is caused by the mutual repulsion of the electric field of each electric charge in the course of the transfer of the electric charges through the charge transport layer. However, in the case where the thickness of the charge transport layer is reduced, the surface potential of the photoconductor decreases if electric charges are applied thereto under the same conditions, sufficient satisfactory development for use in practice cannot be carried out. Some conventional photoconductor with such a reduced charge transport layer is not capable of clearly reproducing low-contrast thin line images, or completely unable to produce such line images in some cases.
In such conventional photoconductors for analog writing, the undercoat layer has such a thickness that is sufficient for preventing charge injection from the electroconductive support into the charge generation layer or the charge transport layer, or for making it possible to perform pretreatment to the electroconductive support for uniformly coating the CGL on the electroconductive support. Therefore, the UL is extremely thin, with such a thickness that is negligible in comparison with the thickness of the CTL.
Furthermore, even when an LED is used for digital writing, the light from the LED is incoherent light, so that in the function-separated type layered photoconductor for use with the LED, it is unnecessary to cause the UL to have a light scattering function, and therefore, even in this case, the UL has such a thickness that is negligible in comparison with the thickness of the CTL.
In the conventional function-separated type layered photoconductor either for analog writing or digital writing, as long as incoherent light is used for writing, thin line images which are recorded with a relatively small amount of energy, cannot be reproduced with high image formation performance by merely decreasing the thickness of the CTL.
It is therefore an object of the present invention to provide an electrophotographic image formation apparatus capable of preventing the formation of blurred images, with excellent image formation performance and accordingly with excellent image development performance.
This object of the present invention can be achieved by use of a function-separated type photoconductor in an electrophotographic image formation apparatus, which function-separated type photoconductor comprises an electroconductive substrate on which an undercoat layer, a charge generation layer and a charge transport layer are successively overlaid, to which undercoat layer high voltage resistance is imparted by increasing the thickness thereof, thereby imparting high surface potential to the photoconductor.
More specifically, the present invention can be achieved by an image formation apparatus comprising:
a photoconductor capable of forming a latent electrostatic image thereon,
charging means for charging the surface of the photoconductor,
image exposure means for exposing the charged surface of the photoconductor to a light image corresponding to an original image to be reproduced, through an optical lens, using part of light to which the original image is exposed, and
development means for developing the latent electrostatic image to a visible image, wherein the photoconductor is a function-separated type photoconductor comprising (a) an electroconductive support, (b) an undercoat layer with a thickness of Tul formed on the electroconductive support, (c) a charge generation layer formed on the undercoat layer, and (d) a charge transport layer with a thickness of Tctl, which is 20 xcexcm or less, formed on the charge generation layer, Tul and Tctl satisfying a relationship of Tul greater than Tctl/3.
In the above image formation apparatus, it is preferable that Tul and Tctl satisfy a relationship of Tul greater than Tctl/2.
The object of the present invention can also be achieved by an image formation apparatus comprising:
a photoconductor capable of forming a latent electrostatic image thereon,
charging means for charging the surface of the photoconductor,
image exposure means for exposing the charged surface of the photoconductor to a light image corresponding to an original image to be reproduced, using incoherent light, with the light image being divided into picture elements, and
development means for developing the latent electrostatic image to a visible image, wherein the photoconductor is a function-separated type photoconductor comprising (a) an electroconductive support, (b) an undercoat layer with a thickness of Tul formed on the electroconductive support, (c) a charge generation layer formed on the undercoat layer, and (d) a charge transport layer with a thickness of Tctl, which is 20 xcexcm or less, formed on the charge generation layer, Tul and Tctl satisfying a relationship of Tul greater than Tctl/3.
In the above image formation apparatus, it is preferable that Tul and Tctl satisfy a relationship of Tul greater than Tctl/2.
The present invention can also be achieved by an image formation apparatus comprising:
a photoconductor capable of forming a latent electrostatic image thereon,
charging means for charging the surface of the photoconductor,
image exposure means for exposing the charged surface of the photoconductor to a light image corresponding to an original image to be reproduced, using coherent light, with the light image being divided into picture elements, and
development means for developing the latent electrostatic image to a visible image, wherein the photoconductor is a function-separated type photoconductor comprising (a) an electroconductive support, (b) an undercoat layer with a thickness of Tul formed on the electroconductive support, (c) a charge generation layer formed on the undercoat layer, and (d) a charge transport layer with a thickness of Tctl, which is 20 xcexcm or less, formed on the charge generation layer, Tul and Tctl satisfying a relationship of Tul greater than Tctl/2.
In the above image formation apparatus, it is preferable that Tul and Tctl satisfy a relationship of Tul greater than Tctl.
The present invention can also be achieved by an image formation apparatus comprising:
a photoconductor capable of forming a latent electrostatic image thereon,
charging means for charging the surface of the photoconductor,
image exposure means for exposing the charged surface of the photoconductor to a light image corresponding to an original image having an image data including gradation information to be reproduced, using incoherent light, with the light image being divided into picture elements,
development means for developing the latent electrostatic image to a visible image, and
gradation representing means for representing an image formation method based on gradation, wherein the gradation representing means is capable of inputting a driving signal to the image exposure means for controlling the image exposure means, based on the image data of the original image, the driving signal having a predetermined minimum value, the photoconductor being a function-separated type photoconductor comprising (a) an electroconductive support, (b) an undercoat layer with a thickness of Tul formed on the electroconductive support, (c) a charge generation layer formed on the undercoat layer, and (d) a charge transport layer with a thickness of Tctl, which is 20 xcexcm or less, formed on the charge generation layer, Tul and Tctl satisfying a relationship of Tul greater than Tctl/2, the photoconductor having such a differential sensitivity that is not more than ⅓ a maximum differential sensitivity of the photoconductor at a maximum exposure in an exposure distribution on the surface of the photoconductor at the predetermined minimum value of the driving signal, wherein the exposure distribution is represented by E(x,y) [joule/m2] calculated by integrating a writing light energy distribution P(x, y, t) [watt/m2] with respect to an exposure time (t), where (x, y) is a coordinate on the surface of the photoconductor, and an exposure diameter Db satisfying a relationship of Db greater than Tctl, wherein the exposure diameter Db is a smaller exposure diameter of an exposure diameter measured in a main scanning direction or an exposure diameter measured in a sub-scanning direction in an area on the surface of the photoconductor at xc2xd the maximum exposure or more in the exposure distribution on the surface of the photoconductor at the predetermined minimum value of the driving signal.
In the above image formation apparatus, it is preferable that Tul and Tctl satisfy a relationship of Tul greater than Tctl.
The present invention can also be achieved by an image formation apparatus comprising:
a photoconductor capable of forming a latent electrostatic image thereon,
charging means for charging the surface of the photoconductor,
image exposure means for exposing the charged surface of the photoconductor to a light image corresponding to an original image having gradation information to be reproduced, using coherent light, with the light image being divided into picture elements,
development means for developing the latent electrostatic image to a visible image, and
gradation representing means for representing an image formation method based on gradation, wherein the gradation representing means is capable of inputting a driving signal to the image exposure means for controlling the image exposure means, based on the image data of the original image, the driving signal having a predetermined minimum value, the photoconductor being a function-separated type photoconductor comprising (a) an electroconductive support, (b) an undercoat layer with a thickness of Tul formed on the electroconductive support, (c) a charge generation layer formed on the undercoat layer, and (d) a charge transport layer with a thickness of Tctl, which is 20 xcexcm or less, formed on the charge generation layer, Tul and Tctl satisfying a relationship of Tul greater than Tctl/2, the photoconductor having such a differential sensitivity that is not more than ⅓ a maximum differential sensitivity of the photoconductor at a maximum exposure in an exposure distribution on the surface of the photoconductor at the predetermined minimum value of the driving signal, wherein the exposure distribution is represented by E(x,y) [joule/m2] calculated by integrating a writing light energy distribution P(x, y, t) [watt/m2] with respect to an exposure time (t), where (x, y) is a coordinate on the surface of the photoconductor, and an exposure diameter Db satisfying a relationship of Db greater than 2 Tctl, wherein the exposure diameter Db is a smaller exposure diameter of an exposure diameter measured in a main scanning direction or an exposure diameter measured in a sub-scanning direction in an area on the surface of the photoconductor at xc2xd the maximum exposure or more in the exposure distribution on the surface of the photoconductor at the predetermined minimum value of the driving signal.
In the above image formation apparatus, it is preferable that Tul and Tctl satisfy a relationship of Tul greater than Tctl.